fix(dojo): make mobile chat drag-to-close reachable

## Problem

`useMobileChat` clamps `chatHeight` to 50–100%, then closes on mouseup only when `chatHeight < 50`. Because the stored height never falls below 50, the close-and-reset branch is unreachable and dragging downward cannot close the mobile chat.

## Expected behavior

Dragging below the close threshold closes the mobile chat and resets it to `defaultChatHeight`, while the rendered panel remains constrained to its intended height range.

## Verification

Add focused coverage for the close threshold and reset behavior.

Discovered while reviewing #2460; this is pre-existing and separate from that PR's lint-only scope.